Aircraft structure



Uct. 16, 1945. B. N. WALLIS AIRCRAFT STRUCTURE 2 Sheets-Sheet l FiledJune 17, 1942 B. N. WALLIS AIRCRAFT STRUCTURE 0ct. "l6, 1945.

Filed June 17, 1942 2 Sheets-Sheet 2 Patented Oct. 16, 1945 AIRCRAFTSTRUCTURE Barnes Neville Wallis, Weybridge, England, as- "sighor toVickers-Arm'strongs' Limited, Westminster, London, England ApplicationJune 17, 1942, Serial No. 447,406 1 I In Great Britain August 8, 1941 6Claims.

The invention has reference to the construction of stressed skinstructuresfor aircraft fuselages and Wings. It has heretofore beenrthecustom to divide the surface of the outer plating into a series ofrelatively small panels by riveting or welding the plating to closelypitched longitudinal spars or stringers and transverse ribs orintercostal members in order to enable the relatively thin plating todevelop a high proportion of the inherentshear strength whichit iscapable of developing in bulk form. The attachment of the plating to theframes at frequentintervals serves to stabilise the plating against theformation of wrinkles which tend to occur in thin plate under the actionof shear stress, but this form of construction is disadvantageous inthat the multiplicity of riveted joints involves a large amount ofskilled labour, while the use of a large number of closely pitchedlongitudinal members results in these members being correspondingly thinin cross-section with consequent instability and reduction of thecompressive strength which the material is capable of developing in bulkform, as a result of which the overall weight of the structure isappreciably increased.

In the case of an aircraft fuselage (or a restricted portion thereof)which is supercharged to compensate for the low external atmosphericpressure prevailing at high altitudes, there is the added disadvantagethat local bulging of the skin is caused by the high pressure diiferencewithin and without the structure; such bulging not only impairs theaerodynamic qualities of the fuselage, but also originates stresses inthe rivets at the points of attachment of the plating to the structuralmembers. thereby starting the joints, producing air-leaks and weakeningthe structure.

Alternative constructional methods have been proposed, consisting inattaching the plating either to the longitudinal members only, or to thetransverse members cnlv. but the structural difliculties have not beenremedied, because in each case the plating is still free to bulge, i. e.either fore and aft between the longitudinal members, or transverselybetween the ribs, so that the tensile leading upon the rivets isaccentuated rather than relieved and the bulges in the plating becomedisproportionately more pronounced by the reduction in the number ofpoints of its attachment to the skeletal structure. The firstalternative of attaching only to the longitudinal members does nothavethe effect of reducing the number of transverse members as these arerequired in any case to support the slender longitudinals. The secondalternative of attachment to transverse members only is particularlydisadvantageous in the. case of pressure cabins, since it, gives rise toan ,eX- aggerated form of the worst kind of bulging, when the furrowsare atfright angles to the air stream. In neither case is the totalamount of riveting required very greatly reduced.

The object of the present invention is to provide an ,improved method ofconstruction, where by (a), the longitudinal members may be reduced innumber to the, minimum, to enable them to be made of thick and robustcross-sectional proportions and consequentlly improve their ability towithstand compressiveloading, whilst also permitting a correspondingreduction in the riveting of plating to longitudinals, (b) the rivetingof the transverse members to the plating (previously considerednecessary), may be entirely'eliminated, and (c) in-a.pressure cabin bothforms, of bulging will be precluded. This method improves the structural'efficiency by achieving a considerable reduction 'in overall weight,and uses proportionately less labour ,during construction. Afultherimportant ad'vanf tage possessed by the invention lies in theelimination of the bulging of the plating in thecase of 'the uppersurface of wingswhere the pres sure distribution is below the staticpressure of the atmosphere in the locality. g

In a fuselage or wing structure in accordance with the invention, theskin plating is attached to longitudinal members between which it isstretched over shaped ribs in such manner that it is subjected to acircumferentialtensild stress running in the direction of the planescontaining the 'ribs, andso that said ribs also serve to support, theplating against inwardcollapse by strong centrifugal pressure againstthe plate, there being however no attachment of the plating'to saidribs. v f 7 j The imposition of centrifugal pressure upon the plating inthis manner enables it to withstand a high degree of shear stress, andthereby to resist the shear and torsionalloads to which the structure issubjected in use. In the ap'plication of the invention to a non-pressurecabin or fuselage, or to a wing, the plating remains tightly heldagainst the supporting ribs, but in a pres- I sure cabin theabsence ofany attachmentofthe platin tosaidribs permits the skin together with thelongitudinal members to which it is at,-

tached, .to breathe by free cylindrical expansion shear stress increasesas the internal pressure rises in relation to the external atmosphericpressure, so that when such dilatation occurs the inward support of theribs is no longer required, although the ribs are ready to resume theirduty of supporting the plating against inward collapse when the internaland external air pressures are equalised; p

The invention :is hereinafter more fully described with reference to theaccompanying drawings, which illustrate byway of example the applicationof the invention to the construction of a pressure cabin for use in anaircraft intended for flight at high altitudes; In said drawings,

3 are diagrammatic views showing the relative dispositions of thecomponent sections of the fuselage during the process of theirassembly, 1. e.

respectively as seen before and after the longeron channel-membersare.bolted together. (In Fig. 2 extent tol which the sections ,have revertedfrom the forminwhichitheyr were held duringsthe riveting of the ribstoithe channel-members, and whichthey resumewhen boltedtogether as showninILig. '3, vhasbeen-rexaggerated forthe purpose of illustration). Fig.4is a detail view depicting the manner in which the ribs are.locat'edrlaterally in .relation to theskin plating, andrFig. 5 is afragmentary perspectiveillustration of ajig. suitable for. the purposeofimposing circumferential stress ,inthe plating whilst clamping the ribs.and channelmembersin position= for their attachment.

The structureofwth fpressure cabini1lustrated Lin-the drawings"issdivided longitudinally intosthree-segmentsh, .B: and. G of equal arcdimension,- and therpresentsdescription is confiiiedrt'o astructure OfthiS kind, although it will be: understood that the. inventimr isnot iso restrictedinitsapplication. I

.Theilongerons.larecomposed-of pairs of channel-section members .-6, i,respectively attached to the opposingedges ofsadjacent segments of thefuselage; the individualsmembers ofeachvpair be ingboltedtogether-duringthe assembly of the separately constructed segments, sothat they combine :to form" completeelongerons. Alternativeliangle-bars.may employed :lieu of channel-section members, if desired. Thestiffening-ribs, whichare spaced along thelength of thesfuselage, areeachconstituted by three individual arcuate segments '7 fabricated fromLesect'ion-barsirolled to the appropriate curvature and=providedalongtheedge of the outer flange with a cushioning-or anti-chafingflpador head 8 of vulcanized fibre; india-rubberor other suitablenon-metallicmaterial. Itwillbeunderstood'that the-number of: the arcuatesegments which constit-ute the stiffening-ribs. at eac'h cross-sectionalpitch will correspond with the numbenof longitudinalsections which-areto compose the-structure. .Beforeproceedingto a:descripti0n of theerection of; thefuselage, there follows anv explanationof.our-.preferredimethod ofwmanufactmeh flone of thecomponenhsectionsthereof. The plating of arsectionalunitisi'first formed from #8.. sheet-9 which; after it-has been wrought-to the requisite cylindrical orpart-spherical curvavture, is riveted at 9l .along each =ofritslongitudinal borderstoone the flanges: I of one of: saidchannel-membersei, so "that the plating lies upon the .outs'idefaceofsaid flange, thefree edge of which is directed towards themiddle-oftheplate.

AJongitudinal reinforcing Plate Illis preferably interposed between theflange 6] of the channelmember 6 and the underside of the plating 9along the boundary of the latter, in order to assist in distributing thestresses to be transmitted between th plating and the longerons. Thesaid reinforcing plates I!) may have tongues I01 extending inwardlywherever a segmental tiffen- 'then supported upon a jig comprising twomembers having surfaces which, as in the present case of a fuselage tobe erected from three component sections of equal arc dimension, arerelatively inclined-at an angle of One of the two members of the jig isindicated at J in Fig. 5, anditwill'be seen that the outwardly directedback-0f the web of one of the channel-members 6 is constrained to lieflush against the inclined surface J l of said jig member J by means oftemporary servicerbolts ll. Said bolts H are passed through pads 52which are riveted between the flanges of. the channel-members. The otherchannelem'ember 6 of the sectionis bolted insimilar fashion to the otherjig member (not shown). It will be understood that the:re1ativeinclination of. the surfaces J of the jig members J will .be varied tosuit the case of a three-section fuselage of which the sections are notall of the same arcrdimension, or. to suit fuselages or wings composedof a difierentnumber of sectionalunits. r Thesegmental stiffening-ribs!required for the unitunder construction, being provided at theirextremities with gusset-plates H and 12, are assembled-tin parallelarrangement beneath-the platingi}, with their ends suitably disposed inrelation to the flanges of the channel-members 6,.and withethe partsassembledin this conditionseachirib l is subjected to an endwiseforceso.as-.topress it outwards to the required degree. The Tmeans employedtoimposesaid force upon the 'rib comprises; inassociation. with 143E611jig, anIsbutment-bar vl? detachably fitted transversely through aehole"l3 -drilled the web of the-rib near its extremitm andtwocranked:levers--Q,Q whicharesarranged so that they bear-with their toesQ upon the pads 52 as closely-as possible to the underside of the outerflange-5i of the channel-member -6; whilsttheir otherrendsare pressedupwardly againstrsaid bar Pfby operation ofset-screws R, R. The eifcctof"-tighteningthe set-screws 1% against the face -of'th-e jig member Jisthusto impose astrong outward pressure uponrthe arouate rib-'1',through the -medium of thesabntment bars P, whilst the reaction of theoutward-pressing .forceserves to intensify the pressure of the web facesof the -channel-members 6 against the inclined jig-surfaces'd Theoutward motion of the ribs i is resisted by the plating S attachedbetween the members'ii; and'a tension is thus set up in the plating 9 bythe radial outward pressure of the ribs '1', the 'degree of suchrtensionbeing readily determinedby adjustment of. thema-gnitude of the-endwiseforce impressed upon-the ribs?! by the 'set-screws"R. In this.condition'ofthe parts; with the plating 9. stressed at the appropriatecircumferential tension,.holes aredrilledin thechannel-members'fitoregister with holes-1e already drilled in the gussetsliiand 12,:and'theg-ussets are then'rrveted to the members 6, which latter arethereby securely attached to the ribs 1, before the setscrewsRarereleased.

, c When .theunitsection' is-removedfrolnthe jig, it revertspartially-from the arcuate cross-sectional form in which it has beenheld by the pres-- sure of the levers Q against the bars P, the tensionin the plating Qbeing reduced by a slight straightening of the ribs 1until a balance of forces is attained.

Three sectional units A, B and C constructed in this manner are thenassembled with the outer faces of the webs of their channel-members 5 injuxtaposition. It will be evident that, owing to the aforesaid reversionin shape of the units when released from the jig, the outer faces of thechannel-members 6 are no longer inclined at an angle at which two suchmembers can make flush contact to constitute a single longeron; ratheris the disposition of said channel-members 6 relative to one another,substantially as shown in Fig. 2, in which the faces of the members 6gape to a greater or less degree. The requisite condition is howeverrestored by bolting said pairs of channelmembers 6 together by-means ofbolts l2 situated near to the outer edge of the web of the longeron,said bolts l2 being tightened until the opposing faces of thechannel-members 6 are uniformly forced into contact when the fuselagere-assumes a circular cross-section as depicted in Fig. 3; in thismanner the circumferential tension, to which the plating 9 was subjectedin the jig, is re-imposed in the complete structure,

The plating 9, being now pressed strongly against the rib members I, isprevented from buckling inwards should the application of shear ortorsional loads tend to make it do so by passing the limit of stabilityof the platein the free state. By this means the thin plating is enabledto develop the same degree of shear stress as ifit were actuallyattached to the rib members by rivets or bolts, but it will beunderstood that no mechanical connection between the plating and thesegmental ribs does in fact exist, so that the plating is not heldagainst cylindrical expansion if and when the pressure of the internalatmosphere of the fuselage is raised above that of the surrounding air.

Thus, should the internal pressure of the fuselage rise to such a degreethat the resultant hoop stress in the plating is equal to or greaterthan that imposed during construction by a method such as thathereinbefore described, the plating is free to expand to a greater sizethan the internal skeletal structure, by pure cylindrical dilatation,and no bulges are formed such as those which would result from amechanical attachment of the plating and the ribs or stringers. In orderto locate the ribs 1 in their correct positions in relation to thelongitudinal axis of the fuselage, pairs of cleats or the like I 3 maybe provided, which are fixed upon the inner face of the plating 9 andbetween which the ribs 1 are free to slide when cylindrical expansion ofthe skin occurs.

The joints between the adjoining faces of the channel-members of thelongerons are rendered air-tight by the interposition of strip rubber orother suitable jointingmaterial H! before they are bolted together.

The method of construction hereinbefore de scribed is equally suited tothe construction of non-pressure cab-ins and fuselages and wings, thecustomary ovoid or elliptical cross-section being achieved by means ofsuitably curved internal stiffening-ribs, or by suitably disposing the1ongerons.

An aircraft wing manufactured in accordance with the invention isconveniently formed in two component sections, each of which is sodisposed along its'leading and'trailing edgesas-to form 'ahalf-longitudinal member, one section forming the upper part of the wingand the other section the lower part thereof, and the arrangementbeingsuch that when the corresponding edges of the sections are joined,they combine to compose the leading and trailing longitudinal spars ofthe wing. As in the method of construction previously described, the actof bolting said halfmembers together imparts the requisite trans-1 versetensile stress to the skin plating which is stretched between thesparsover the arcuate internal ribs. Additional main spars may he intro-.duced into the wing, in which case they may be utilised to maintain thecorrect loc'ation'of the ribs and thereby permit theaforesaid cleats tobe dispensed with. J It will be evident that the method of con-'struction herein described possesses the additional advantage that itis possible to complete the attachment of all fittings and equipment 611the interior of a sectional unit before it is assembled for erectionwith the other component units, the only operations remaining to'becarried out after assembly being the connection of pipe lines, electricleads and the like; In'this manner it is possible to avoid the wastageof time occupied in inserting such fittings and equipment which wouldotherwise occur by reason of the restricted space within the structureand the con sequent difficulty of access to its interior and the limitednumber of persons who can be em-' ployed within the structure at thesame time.

What I claim as my invention and desire to secure by Letters Patent is:,1

1. The method of making and'assembling stressed skin structures foraircraft or the like, which comprises warping a'sheet of skin plating toan appropriate curvature, anchoring the sheet at its opposite edgesrespectively to two longitudinal elements of the skeletal portion ofsaid structure, applying transverse skeletal elements at spacedintervals against the inner surface of the skin with their ends abuttingsaid longitudinal elements at intervals therealong while maintainingsaid skin in contact with but free from connection with said elements,stretchin said skin over said transverse elements without attachmentthereto, and securing the ends of said transverse elements to theirrespective adjacent longitudinal elements while the skin is stretched.

2. The method of making and assembling hollow substantially tubularstressed skin structures for aircraft or the like which comprisesforming longitudinal segmental sections of said tubular structure byanchoring two longitudinal skeletal elements, having a substantialradial dimension, in the approximate spaced apart positions they willoccupy in the completed structure, warping a sheet of skin plating tothe desired curvature of said section, and securing it along itsopposite longitudinal edges to the radially outward edges of saidlongitudinal elements, applying a series of longitudinal spaced curvedribs against the inner surface of said sheet without attachment thereto,with the ends of said ribs disposed adjacent said longitudinal elements,applying endwise compression to said ribs whereby they are caused topress radially outwardly against said sheet, and securing the ends ofsaid ribs while thus stressed to the longitudinal elements, releasingthe stress on the ribs, removing said longitudinal elements from theiranchored position, whereupon they will be sprung to a somewhatnon-radial position as the tension in the attached plating and thecompression: in the adjacent ribs are, i balanced; thenysecuringthe-segmental sections: together edgeto edge, drawing the, longitudinalelements together-and-gjoining them firmly, whereby they again assumezit-substantially: radial position: and the previous tension in, theplating is reimposed.

3; A pressure cabin for highaltitude aircraft or-the-like, ofsubstantially circular cross section, comprising circumferentiallyspaced longerons, 'a plurality of arcuate ribs spaced apart andextending ,betweensaid longrons, skin plating attached to saidlongeronsand applied. over said ribs. in initial and normal contacttherewith but notiattached thereto, said skin plating being inaeondition of circumferential tensile stress and the ribs ina:corresponding-condition of endwisecompression, whereby they exert strongradial outward pressure against the-plating, and-guide means ontheplating to locate the ribs laterally with-respect :thereto but topermit cylindrical dilatation of the skin independently of the .ribs.

4. A stressedskinstnicture for an aircraft fuselage;:wing, or likewalled structure comprising, azhollow, continuous-walled constructionconstitutedby, a iplurality' fr' unitary segmental prefabricatedsections and including, .in combination,

- peripherally spaced longitudinally extending I bracingimembers; eachcomprisinga pair of abutendwise. compression substantially along thechords of thearcs they represent, whereby they exert a continuous-pressure radially outwardly against the sheet which is therefore in acondition of corresponding tension, the firm attachment of the marginallongitudinal elements of adjoining sections serving to maintainthem insubstantiallyradial positions and maintaining the stress in the ribsandsheet;

5. The structure set forth in claim' 4 in which there is additionallyprovided guide means on'the plating to locate the ribs laterally withrespect thereto but to permit relative :movement of the plating over theribsin a circumferentialdirection;

6. An aircraft'wingiof hollow construction comprising-two outwardlybowed complementary sections respectively forming the upper and lowersurfaces'of the wing, said sections being joined at the leading andtrailing edges, each section comprising a. longitudinal.element'adjacent each of said edges, a plurality of spaced arcuatetransversely extending ribssecured between said longitudinal elements ofthe section, saidribs being in a state of endwise compression,substantially along the chords of the arcs which they represent, acurved sheet of skin plating secured marginally to the radially outwardedges of the said longitudinal elements of the section and beingnormally in firm'contact' with the outer surfaces of said arcuate ribswhereby said ribs exert a continuous pressure radially outwardly againstthe sheet whichis thereforein =a condition of corresponding tension, andmeans for rigidly securing together, in abutting relationship, theadjoining longitudinal elements of the two sections at the leading andtrailing edges thereof to compose the wing-spars at the leading andtrailing edges of the wing.

BARNESNEVILLE WALLIS.

